core: Initialize EnumValue::m_value

Avoid UB errors (spotted by sanitizers) such as "runtime error: load of
value 52428820, which is not a valid value for type 'WifiPhyBand'

src/core/model/enum.h

@@ -73,7 +73,7 @@ class EnumValue : public AttributeValue
     bool DeserializeFromString(std::string value, Ptr<const AttributeChecker> checker) override;
 
   private:
-    T m_value; //!< The stored value.
+    T m_value{}; //!< The stored value.
 };
 
 template <typename T>